Add support to stream factory (#2898)

This changeset enables declaring the `stream` config value as a factory
method. Providing a much more flexible control of the socket connection.

Defining a custom `stream` config value allows the postgres driver to
support a larger variety of environments/setups such as proxy servers
and secure socket connections that are used by cloud providers such as
GCP.

Currently, usage of the `stream` config value is only viable for single
connections given that it's only possible to define a single socket
stream instance per new Client/Pool instance. By adding support to a
factory function, it becomes possible to enable usage of custom socket
streams for connection pools.

For reference, see the `mysql2` driver for MySQL (linked below) for
prior art example of this pattern.

Refs: https://github.com/sidorares/node-mysql2/blob/ba15fe25703665e516ab0a23af8d828d1473b8c3/lib/connection.js#L63-L65
Refs: https://cloud.google.com/sql/docs/postgres/connect-overview
Signed-off-by: Ruy Adorno <ruyadorno@google.com>

Signed-off-by: Ruy Adorno <ruyadorno@google.com>

packages/pg/lib/connection.js

@@ -14,7 +14,12 @@ class Connection extends EventEmitter {
   constructor(config) {
     super()
     config = config || {}
+
     this.stream = config.stream || new net.Socket()
+    if (typeof this.stream === 'function') {
+      this.stream = this.stream(config)
+    }
+
     this._keepAlive = config.keepAlive
     this._keepAliveInitialDelayMillis = config.keepAliveInitialDelayMillis
     this.lastBuffer = false

packages/pg/test/unit/connection/startup-tests.js

@@ -7,6 +7,18 @@ test('connection can take existing stream', function () {
   assert.equal(con.stream, stream)
 })
 
+test('connection can take stream factory method', function () {
+  var stream = new MemoryStream()
+  var connectionOpts = {}
+  var makeStream = function (opts) {
+    assert.equal(connectionOpts, opts)
+    return stream
+  }
+  connectionOpts.stream = makeStream
+  var con = new Connection(connectionOpts)
+  assert.equal(con.stream, stream)
+})
+
 test('using any stream', function () {
   var makeStream = function () {
     var stream = new MemoryStream()